This was the product of a card that, while solid in terms of fights, featured almost no one of significant name value.
Josh Barnett may be a former UFC champion, but that was well before the "boom period" for the sport and he has had basically no fights of consequence for casual fans in years. Nobody but hardcore MMA fans know who Sergei Kharitonov is. Daniel Cormier is a rising star but he's also a nobody in terms of being a draw.
The Strikeforce brand is simply not big enough to be a draw on its own and Zuffa provided very little support in pumping up the event, instead (understandably) focusing their PR on the much bigger news of the upcoming Fox event and the news that Brock Lesnar would return and face Alistair Overeem.
So an under-supported card featuring no names for a promotion that is clearly being picked over by its "big brother" drew terrible ratings. I suppose that shouldn't be a shock. But seeing that Bellator was almost able to pull off the same numbers is still pretty amazing.